Moving media information recording and reproducing devices, such as magnetic tape and magnetic and optical disc recorders and players, sometimes must synchronize their operations precisely with external signals or with other such devices. For example, in a television recording studio, the playback of video information must be synchronized precisely with a studio reference signal to facilitate transient-free switching between multiple sources of video. When audio or video information recorded upon moving magnetic media is edited, it is essential to synchronize precisely the operation of the source of the information with that of the machine upon which the information is to be recorded, again to assure transient-free transitions between information recorded previously and newly-recorded information added through editing.
Helical scan video tape recorders have a separate control signal track recorded at the edge of the tape that can be used for synchronization purposes. The more recent of these recorders record the information in digital form. The audio and video information is packaged into sectors which are recorded onto diagonal tracks that extend across the tape. The information flow rate is so great that pairs of closely spaced heads mounted upon a rotating scanner are used to record two tracks simultaneously. As each pair of tracks is recorded by these moving scanner-mounted heads, a separate, stationary recording head records pulses onto the control signal track that mark the locations of pairs of tracks.
When such a video tape is played back, a capstan servo system is used to adjust the speed and phase of the playback capstan drive by comparing the timing of the pulses reproduced from the control signal track with the timing of pulses that are synchronized with a studio reference signal or other stable reference. The servo mechanism adjusts the frequency and the phase of the capstan drive so that the speed of tape motion during playback is precisely that needed to synchronize the control signal track pulses with the stable reference pulses.
Under certain circumstances, this provides insufficient precision and further correction is needed. The spacing between the helically scanned diagonal tracks is so small that a capstan servomechanism controlled solely by control signal track information can sometimes position the tape so that the scanning pair of heads is positioned over and scanning the wrong pair of tracks. This mispositioning of the scanning heads can be detected after the digital information recovered from the sectors on the tape is fully decoded, error corrected, and analyzed, because each sector within each track contains information that identifies what information the sector contains and its position. Appropriately designed control circuitry can detect and correct this positioning error only after the digital information is fully recovered.
This technique for achieving proper playback synchronization can be utilized only if the tape playback system includes a complete set of playback signal processing elements for recovering the information, typically including circuitry for generating a playback timing signal and synchronizing it with the actual playback signal, circuitry for demodulating the encoding of the information flowing from the tape (which may have been encoded, for example, using the channel code as described in Miller U.S. Reissue Pat. No. Re. 31,311, which code is hereinafter referred to as the Miller Channel Code), circuitry for unshuffling the information if it has been shuffled, circuitry for combining the information flowing from the pairs of tracks that are scanned simultaneously, and circuitry for performing error detection and correction (as by using well known Reed-Solomon error correcting codes). Collectively, these playback signal processing elements are hereinafter called a "playback signal processing unit."
Situations arise where it is desirable to achieve playback capstan synchronization but it is not necessary to recover the information. In such situations it would be desirable to be able to achieve such synchronization without the need of such a playback signal processing which would otherwise be required. For example, when using multiple tape transports to duplicate tapes or to provide for rapid switching between multiple queued-up tapes, it would be less expensive to provide only one playback signal processing unit which can be shared by all of the multiple tape transports, connecting it to the playback signals produced by whichever output is to be transmitted or monitored, rather than to equip each transport with its own independent playback signal processing unit. However, in prior systems if only one playback signal processing unit were provided, then when switching a single playback signal processing unit from the output of a first machine to that of a second machine, there would be no way to synchronize the output of the second machine with that of the first. That is, because the second is not connected to the playback signal processing unit prior to the moment of switching, it cannot be fully synchronized with that of the first. There would be no assurance that a switching transient would not occur when the playback signal processing unit was switched from the output of the first machine to that of the second.
Other problems arise in editing or tape duplication situations, when new material is to be added to a tape containing existing material, and a smooth, transient-free transition between the new and existing material is desired, and particularly where entire sectors are to be transferred intact from one magnetic tape to another, possibly without disturbing other sectors on the target tape (as when the video sectors are replaced and the audio sectors are not replaced, or vice versa). Because the record and playback head pairs are mounted 90 degrees apart from each other on the rotating scanner, the playback heads are not adjacent the record heads and therefore may not be used to position the record heads for recording. Therefore, playback information extracted from the record heads (used as playback heads for this purpose) must be used to check that the record heads are properly positioned over the correct tracks. Unfortunately, the record head gaps are conventionally wider than the playback head gaps and cannot reproduce the highest recorded frequencies properly. Thus even where a playback signal processing unit is available and connected to the record heads (used as playback heads for head positioning purposes), the playback signal may be so badly degraded by the poor frequency response of the record heads (when used for playback) that the playback signal cannot be properly decoded and used to check the head positioning.
It is also practical in some editing situations to transfer digital information encoded into sectors directly from one tape to another tape without fully decoding the information but keeping it intact as complete sector units. In such a case, it would not be necessary to provide a playback signal processing unit unless it were needed for synchronization. In practice, this is how only the video sectors or only the audio sectors can be transferred from one tape to another. To carry out such a transfer of complete sectors from the playback heads of one machine to the record heads of another, it is essential that the playback machine scan its tracks and sectors (with its playback heads) nearly synchronously with the record machine's scanning of its tracks and sectors (with its record heads). Absolute precision in synchronization is not essential because the positioning of any given sector within a track may be changed slightly without adverse consequences due to the edit gaps that separate the sectors sharing a common track from each other.
In these and other analogous situations, it would be desirable to facilitate the synchronizing of playback machines with a studio reference, the synchronizing of multiple playback machines, or the synchronizing of a playback machine with one or more record machines without the need for a playback signal processing unit.